Routing between ISPs

[anousakisioannis]

Hello,

In our network we have have a lot of sites over the world. In some of them we use ISP_A, in other we use ISP_B and in few of them both ISPs.

As routing protocol we use EIGRP and we have L3 mpls.

My question is how site_A with ISP_A learns routes (for example) from site_B with ISP_B ?

Thank you in advance

[Joseph W. Doherty]

"My question is how site_A with ISP_A learns routes (for example) from site_B with ISP_B ?"

Sites attached to only one provider could learn about sites only to the other provider via your sites that are attached to both providers.

[anousakisioannis]

Do you know if ISPs exchange routing information or use common BGP AS # and this how they reach networks from other ISPs?

Thank you

[paul driver]

Hello

BGP is the main routing protocol of the internet - It a external route protocol used to provide route selection. And is what all Isps use to exchange advertise their networks.

ISPs are in teirs -With the highest tier being internet backbone isps that exchange routes that provide routes to every iso and give you access to anywhere in the world

Isp teir 2s exchange traffic between these internet backbone teir 1 isps and the lower their 3 isps these are usually your main telecoms providers like British telecom here in Britain - these will usually provide network for a teir 3 ips 

Teir 3 isps are you local providers these usually provide you home internet traffic and they that obtain their internet traffic from the likes of a teir 2 isp
So bascaiy in a nutshell your home internet is from a teir 3 isp address range which is part of a teir 2 adress range etc,

So in summary each isp will in some way exchange its routes with other isps between it own teir or below or above it.

res

paul 

[anousakisioannis]

Hello Paul,

Thank you for your answer. It's more clear now..

Do you have any technical details how they exchange the routes? Do they use common BGP AS # to exchange the routes?

Regards